GENEVA — European researchers say they have discovered a new subatomic particle that helps confirm our knowledge about how quarks bind — one of the basic forces in the shaping of matter.

The CERN physics research center said Friday that the particle was discovered at the Compact Muon Solenoid, one of the Large Hadron Collider's two main general-purpose detectors, in collaboration with the University of Zurich.

Joe Incandela, the physicist in charge of the experiment involved with the discovery, told The Associated Press that the particle was predicted long ago, but finding it was "really kind of a classic tour de force of experimental work."

The particle, known as an excited neutral Xi-b baryon, could not be detected directly because it was too unstable. Instead, its existence was inferred by the pattern of its decay into other subatomic particles.

The Xi-b particle, like other baryons such as protons and neutrons, is made up of three quarks. Protons and neutrons are combinations of "up" and "down" quarks (two up and one down for protons, two down and one up for neutrons). In contrast, the newly detected Xi-b particles consist of an up, strange and bottom quark. The particles are electrically neutral, with a spin of 3/2 and a mass comparable to that of a lithium atom, University of Zurich researchers said.

Xi-B baryons have been previously detected in their ground states, but the particles created in the LHC's proton-on-proton collisions are the first to be observed in their excited states. They're also the first newly discovered particles to be reported by the Compact Muon Solenoid collaboration, which takes in thousands of researchers.

The University of Zurich said 21 of the Xi-b decay events were detected during a series of collisions at an energy level of 7 trillion electron volts last year. Those events were enough to determine that the decay events were more than a statistical fluke.

"The discovery of the new particle confirms the theory of how quarks bind and therefore helps to understand the strong interaction, one of the four basic forces of physics which determines the structure of matter," the university said in a news release.

CMS Collaboration / Univ. of Zurich

This schematic shows how the excited neutral Xi-b baryon (bottom left) decays into other subatomic particles that could be detected at the Large Hadron Collider's Compact Muon Solenoid.

CMS physicist Vincenzo Chiochia, one of the co-leaders of the search, told the Symmetry Breaking blog that "finding this complicated decay in such a messy event makes us confident in our abilities to find other new particles in the future.”

Another experimental group at the LHC, using the ATLAS detector, reported its first new particle last year. That particle, known as the Chi-b (3P), consists of a bottom quark and its antimatter equivalent.

Physicists expect to find a wide variety of subatomic particles consisting of various combinations of quarks, but their prime target is the Higgs boson, a different type of fundamental particle that is predicted by theory but has not been detected. If it exists, the Higgs boson would help explain why some particles have mass while others don't.

CERN officials have said they expect the LHC to provide evidence of the Higgs boson's existence or non-existence by the end of this year.

The Standard Model of particle physics is one of science's most successful theories, enabling the development of devices ranging from light bulbs, to microwave ovens and television, to quantum computing devices. The Standard Model is also one of the oddest theories, because it lays out a dizzying menagerie of hundreds of subatomic particles. At its heart are 16 types of elementary particles ... plus at least one more mysterious particle that scientists are spending billions of dollars to detect.

Click on "Next" to get the full rundown.

Quarks

Berkeley Lab

Six "flavors" of quarks have been detected: up and down, charm and strange, top and bottom. Quarks are almost always found in different combinations, bound together by gluons (more on those later). Particles built up from quarks and gluons are called hadrons. The Large Hadron Collider is so named because it's a large collider that smashes hadrons together.

Three-quark combinations fit in the category of baryons. The best-known baryons are the proton (with two up quarks and one down quark) and the neutron (with two down quarks and one up quark).

Particles that have one quark and one antiquark fit in the category of mesons. For example, the pion, or pi meson, contains an up quark and an anti-down quark.

Six "flavors" of leptons have been detected: The negatively charged electron is the best-known lepton — along with its antimatter counterpart, the positron. This photo shows the path of single electrons passing through liquid helium, in an experiment devised by Brown University researchers.

The muon is also negatively charged, but it's about 207 times as massive as the electron. ("Who ordered that?" physicist Isidor Rabi reportedly asked.) The negatively charged tau particle is even bigger — 3,477 times as massive as the electron — but it decays into other particles in less than a trillionth of a second.

Each of those leptons has a neutrino associated with it: the electron neutrino, the muon neutrino and the tau neutrino. Neutrinos interact only weakly with other particles, and they zip through our planet virtually without a trace. Physicists only recently determined that they have mass, but there's still a great deal of mystery surrounding the ghostly particles.

Force carriers

Fermilab

The Standard Model sets aside a category for particles that are associated with force fields. The effect of a field can be viewed as involving an exchange of such force-carrying particles.

Four elementary force-carrying particles have been detected. The best-known force carrier is the photon — which plays a part in the electromagnetic spectrum, including visible light. The gluon binds quarks together through the strong nuclear force. The weak nuclear force involves the exchange of W and Z bosons. The W boson can carry a positive or a negative charge, while the Z boson is neutral.

If gravity could be incorporated into the Standard Model, the force-carrying particle would be called the graviton (shown here in an artist's depiction). However, gravitons have not yet been detected, and at least for now, such particles are not accounted for in the Standard Model.

Bosons vs. fermions

Rice Univ. via AIP

All force-carrying particles are bosons, but not all bosons are force carriers. The difference has to do with a property known as particle spin. Particles with a fractional spin value (for example, electrons, protons and neutrons) are fermions. Two identical fermions cannot occupy the same quantum state. This is a property that keeps electrons from collapsing into a jumble, and thus makes chemical reactions possible.

All particles with a whole-integer spin value are classified as bosons, and such particles can occupy the same quantum state even if they're identical. The photon is the best-known type of boson.

Even atoms can be classified as fermions and bosons. This photo shows how atom clouds of lithium-7 (bosons) and lithium-6 (fermions) behave at low temperatures. The bosons collapse into a compact cloud, while the fermions can't squeeze that closely together.

The Higgs boson is the only particle predicted by the Standard Model that has not yet been detected. The Higgs is the main quarry for physicists at the Large Hadron Collider. This image is a simulation of the Higgs' signature as it might appear in one of the LHC's detectors.

The Higgs boson, named after Scottish theorist Peter Higgs, is thought to be associated with a field that endows some particles (such as the weak nuclear force's W and Z bosons) with mass, while leaving the electromagnetic force's photons without mass.

This Higgs field may have played a role at the very beginnings of the universe: Physicists believe that at the highest energies, the electromagnetic and weak nuclear forces were unified, but something led to "electroweak symmetry breaking" as the infant cosmos cooled. That would be why the electromagnetic force and the weak nuclear force are distinct in the current universe. The Large Hadron Collider could shed new light on this mysterious Higgs mechanism.

Why so complicated?

Tim Jones / McDonald Observatory / HETDEX

Hadrons and leptons? Baryons and mesons? Fermions and bosons? Sometimes it seems as if particle physicists set up these classifications just to keep outsiders totally confused. But for researchers, these occasionally overlapping categories are useful for figuring out how different types of particles interact with each other.

In a sense, it's as if we've been talking about the game of chess but have gotten only to the point of naming the different pieces on the board: black pieces and white ones, pawns and knights, bishops and rooks, kings and queens. The real meaning of the game comes out when you start studying how the pieces perform and interact.